They are not aggressive, they get big like Sugar Magnolia says, but as long as they can't eat another fish they are fine. They wont bother corals or any sessile inverts but any shrimp or crab that they can get will be eaten. They are very active so you need a lot of room even for a growout tank, a 48 inch tank is too small for one let alone 3, and you almost never see them as tiny juveniles. Usually they come in a little smaller then a dinner plate, every once in a while I see a 5 inch one, but that is rare. Only once have I seen a tiny one, it was a fantastic find in the sargasso weed that was close to shore and still had all the the fin streamers, I was collecting quarter sized porcupine puffers with a friend and he caught it.
The other thing about them is they are very poor shippers, even from store to home they lose scales, stress out and get all beat up. If you see them in a store I would put them one to a bucket and cover it so they are completely dark, a big bag is normally how they transport them but they panic and thrash about, causing the damage. Finally they are all caught by hook and line, nets just beat them up so much that they are unsellable if they are seined or barrier caught. Watch for infected or irritated mouth and lips, and make sure they eat before you take them home.